Since GPS-satellites transmit coded signals, range-measuring by a receiver may be implemented in different ways, namely:
By evaluating the received code signals (code measuring) or PA1 By evaluating the carrier phases (carrier phase measuring).
Correlated errors may occur in these measurements between the receivers, which e.g. are due to satellite clock errors and changes in the troposphere and the ionosphere. Those errors are determined as corrections (correction values) for the mobile station. In a differential mode (DGPS) of GPS a range correction can be evaluated in a reference station from a comparison between a pseudo range for the reference station (range evaluated by satellite navigation between the reference station and a receiveable satellite) and a known geometrical range between the reference station and the received satellite. The pseudo range measurements of the mobile station are corrected by means of range corrections which are transmitted by the reference station by means of (data)-radio relay. Non-correlated errors may also appear between receivers, e.g. due to effects of multi-path propagation of the signals between the receivers. Those errors should be suppressed as much as possible since they cannot be reduced by the differential correction. For reducing the effects of non-correlated errors the measured value of the carrier phase is used, the measurement noise and multi-path propagation of which is within the mm and cm wavelength range. However, an ambiguity of the number of wavelengths from carrier phase measurement in a detrimental manner results since the measuring of the carrier phase in a phase-locked loop (PLL)-measuring device does not change in a displacement by a random integer number of the phase cycles. A known technique, the so-called "carrier-smoothed code" combines the code-values with the carrier phase values and suppresses herewith measurement noise and effects of multi-path propagation during code measurement. Herewith by means of the code phase measurement the Doppler shift or the range change from the satellite to the receiver, respectively, between two points of time is determined, and the phase ambiguities by means of a code measurement with the smoothed phase are approximately evaluated. The time constant of the filter of the present technique with the carrier-smoothed code is limited by the ionosphere error since this error has a counter-current behavior during code and phase measurement. In the differential operational mode with the carrier-smoothed code a large filter time constant may reduce the non-correlated errors between the reference and the mobile station. However, a large filter constant in a disadvantageous manner also distorts the ionospheric error in the differential correction which per se is correctable so that the ionospheric error may not completely be corrected at the mobile station. With such a method, which is known as the DGPS-navigation method, a determination of the position of the receiver with an accuracy of approximately .+-.2 m to .+-.3 m (95%) may be attained. This accuracy in the case of some applications, e.g. in approach of landing of a commercial aircraft, is not sufficient, in particular in the case where this approach of landing is to be made under socalled CATIIIb conditions, i.e. at a visual range of zero.